In a liquid crystal display device, a classification based on an operating mode for liquid crystal molecules includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode and a field-induced photo-reactive alignment (FPA) mode. A classification based on a driving mode in the device includes a passive matrix (PM) and an active matrix (AM). The PM is classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type based on a production process. A classification based on a light source includes a reflective type utilizing natural light, a transmissive type utilizing backlight and a transflective type utilizing both the natural light and the backlight.
The liquid crystal display device includes a liquid crystal composition having a nematic phase. The composition has suitable characteristics. An AM device having good characteristics can be obtained by improving characteristics of the composition. Table 1 below summarizes a relationship in two characteristics. The characteristics of the composition will be further described based on a commercially available AM device. A temperature range of the nematic phase relates to a temperature range in which the device can be used. A preferred maximum temperature of the nematic phase is about 70° C. or higher and a preferred minimum temperature of the nematic phase is about −10° C. or lower. Viscosity of the composition relates to a response time in the device. A short response time is preferred for displaying moving images on the device. A shorter response time even by one millisecond is desirable. Accordingly, a small viscosity in the composition is preferred. A small viscosity at a low temperature is further preferred.
TABLE 1General Characteristics of Composition and AM DeviceGeneral CharacteristicsGeneral CharacteristicsNo.of Compositionof AM Device1Wide temperature range ofWide usable temperaturea nematic phaserange2Small viscosityShort response time3Suitable optical anisotropyLarge contrast ratio4Large positive or negativeLow threshold voltagedielectric anisotropyand small electric powerconsumptionLarge contrast ratio5Large specific resistanceLarge voltage holdingratio and large contrastratio6High stability to ultravioletLong service lifelight and heat7Large elastic constantShort response time andlarge contrast ratio
An optical anisotropy of the composition relates to a contrast ratio in the device. According to a mode of the device, a large optical anisotropy or a small optical anisotropy, more specifically, a suitable optical anisotropy is required. A product (Δn×d) of the optical anisotropy (Δn) of the composition and a cell gap (d) in the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on a type of the operating mode. The suitable value is in the range of about 0.30 micrometer to about 0.40 micrometer in a device having the VA mode, and is in the range of about 0.20 micrometer to about 0.30 micrometer in a device having the IPS mode or the FFS mode. In the above case, a composition having the large optical anisotropy is preferred for a device having a small cell gap. A large dielectric anisotropy in the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio in the device. Accordingly, the large dielectric anisotropy is preferred. A large specific resistance in the composition contributes to a large voltage holding ratio and a large contrast ratio in the device. Accordingly, a composition having the large specific resistance at room temperature and also at a high temperature in an initial stage is preferred. The composition having the large specific resistance at room temperature and also at a high temperature even after the device has been used for a long period of time is preferred. Stability of the composition to ultraviolet light and heat relates to a service life of the device. In the case where the stability is high, the device has a long service life. Such characteristics are preferred for an AM device use in a liquid crystal projector, a liquid crystal television and so forth. A large elastic constant in the composition contributes to a short response time and the large contrast ratio in the device. Accordingly, the large elastic constant is preferred.
In a liquid crystal display device having a polymer sustained alignment (PSA) mode, a liquid crystal composition containing a polymer is used. First, a composition to which a small amount of a polymerizable compound is added is injected into the device. Next, the composition is irradiated with ultraviolet light while voltage is applied between substrates of the device. The polymerizable compound is polymerized to form a network structure of the polymer in the composition. In the composition, alignment of liquid crystal molecules can be controlled by the polymer, and therefore the response time of the device is shortened and also image persistence is improved. Such an effect of the polymer can be expected for a device having the mode such as the TN mode, the ECB mode, the OCB mode, the IPS mode, the VA mode, the FFS mode and the FPA mode.
A composition having a positive dielectric anisotropy is used in an AM device having the TN mode. A composition having a negative dielectric anisotropy is used in an AM device having the VA mode. A composition having the positive or negative dielectric anisotropy is used in an AM device having the IPS mode or the FFS mode. A composition having the positive or negative dielectric anisotropy is used in an AM device having the polymer sustained alignment (PSA) mode. An example of a liquid crystal composition having the positive dielectric anisotropy is disclosed in Patent literature No. 1 described below.
If the liquid crystal display device is used for a long period of time, a flicker may be occasionally generated on a display screen. The flicker is associated with image persistence, and is presumed to be generated according to a difference between an electric potential of a positive frame and an electric potentials of a negative frame in driving at alternating current. In order to decrease the generation of the flicker, an improvement from a viewpoint of the structure of the device or the component of the composition is tried.